Which was worse, WWI or WWII?
I understand that the death toll in WWI was greater than WWII but I am not going to base anything on that. Instead I want to submit that the major tragedy of WWI was the Bolshevik revolution and that the major tragedy of WWII was the holocaust. That is of course arguable but, given that, I think the answer is clear.
But first let me spell out why the Bolshevik revolution was attributable to WWI. There are two reasons. The first is that Germany deliberately and with malice aforethought transported the exiled Lenin in a sealed train across Europe to Russia. The German High Command rightly assessed Lenin as a major threat to the Russian war effort and therefore facilitated his trouble-making in that country as part of their own war effort.
Secondly, before the Bolshevik takeover, Russia had become a real parliamentary democracy. The Mensheviks were democrats who believed in elections. The overthrow of the Mensheviks during the war by the Bolsheviks was therefore a violent overthrow of democracy -- which resulted in many millions of deaths both then and across the next 70 years -- and also condemned the world's largest country to tyranny and poverty for all that time.
I am the last to minimize the death of 6 million Jews. But it seems clear to me that the Bolshevik revolution was much more widely harmful.